1. Field of the Invention
The invention relates to an abnormality detection apparatus and an abnormality detection method for an air/fuel ratio sensor.
2. Description of the Related Art
An internal combustion engine for a motor vehicle or the like is provided with an air/fuel ratio sensor that outputs a signal that corresponds to the air/fuel ratio of the internal combustion engine on the basis of the oxygen concentration in exhaust gas, and also with an apparatus for determining the presence/absence of abnormality of the air/fuel ratio sensor, for example, an abnormality detection apparatus disclosed in Japanese Patent Application Publication No. 2005-121003 (JP-A-2005-121003).
In the abnormality detection apparatus of JP-A-2005-121003, the presence/absence of abnormality of the air/fuel ratio sensor is determined by the following procedures “1” to “3”. Firstly, as the process “1”, an active air/fuel ratio control in which the air/fuel ratio of the internal combustion engine is periodically fluctuated between the rich state and the lean state is performed. Next, as the process “2”, a parameter that corresponds to the responsiveness of the output of the air/fuel ratio sensor is found on the basis of the output of the sensor during the active air/fuel ratio control, and the parameter is acquired as data for detecting abnormality. Finally, as the process “3”, the presence/absence of abnormality of the air/fuel ratio sensor is determined on the basis of comparison between the acquired data and an abnormality criterion value. Incidentally, with regard to the processes “2” and “3”, in order to more accurately perform the determination as to the presence/absence of abnormality of the air/fuel ratio sensor, it is also possible to adopt a modification in which the foregoing acquisition of data is performed, and the presence/absence of abnormality of the air/fuel ratio sensor is determined on the basis of comparison between an average value of the acquired data and the abnormality criterion value.
Besides, for example, Japanese Patent Application Publication No. 2005-36742 (JP-A-2005-36742) discloses that a condition that the internal combustion engine is in a state in which the amount of intake air is large is set as a condition for monitoring the air/fuel ratio for the purpose of determining the presence/absence of abnormality regarding the output of the air/fuel ratio sensor. The condition that the internal combustion engine is in the large-amount-of-intake-air state is set because during the large-amount-of-intake-air state of the internal combustion engine, the influence of a breakage of the air/fuel ratio sensor or the like clearly appears in the output of the air/fuel ratio sensor. Therefore, if this condition is used as an execution condition for performing the process “2” in the determination as to the presence/absence of abnormality of the air/fuel ratio sensor, it becomes possible to more accurately perform the determination as to the presence/absence of abnormality.
By setting the condition that the internal combustion engine is in the large-amount-of-intake-air state as an execution condition for performing the process “2”, it becomes possible to more accurately perform the determination as to the presence/absence of abnormality of the air/fuel ratio sensor in the foregoing procedure “1” to “3”. However, corresponding to the setting of this condition, the opportunity of executing the process “2” becomes less, and therefore the opportunity of determining the presence/absence of abnormality of air/fuel ratio sensor also becomes less.